Because articular cartilage lacks the ability for intrinsic repair, injuries following trauma persist and can ultimately lead to post-traumatic osteoarthritis (OA) with resulting pain, reduced range of joint motion and severely impaired quality-of-life. Current standards-of-care for cartilage injuries cannot reliably provide sustained clinical improvement, so there is much interest in the development of alternative, biological approaches towards repair/regeneration. Tissue engineering approaches typically involve the recruitment or transplantation of endogenous mesenchymal progenitor cells (MFCs) to the defect site. Under controlled conditions, these cells have the potential to proliferate, differentiate along the relevant musculoskeletal lineage and regenerate damaged tissue matrix. However, the intra-articular environment is often hostile to chondrogenesis;specifically, inflammation within the injured joint can inhibit tissue regeneration. Joint inflammation is mediated by local levels of cytokines such as interleukin (IL)-1 and tumor necrosis factor (TNF)'-a. Although pro-inflammatory cytokines play an important role in the pathogenesis of OA, there is almost no literature on how they affect cartilage repair strategies involving MFCs. The central hypothesis driving this research plan is that inflammation secondary to joint trauma or subsequent surgical intervention can inhibit the regenerative activity of MFCs introduced to the cartilage defect, thus inhibiting repair. During the mentored phase of this plan, additional tools will be learned to better characterize MPC activity both in vitro and in vivo. Specifically, a toolset of reporter constructs will be developed for monitoring the state of human MPC differentiation, characterizing this toolset in the context of chondrogenesis in vitro. This period will also be spent becoming proficient in imaging techniques for non-invasively tracking inflammatory and chondrogenic activity in vivo, using a nude rat model of MPC delivery to cartilage defects. Upon transition to the independent phase (ROD) of this award, inhibitors of pro-inflammatory cytokine activity will be evaluated for their capability to rescue the chondrogenic behavior of MPCs in vitro. Promising regenerative regimens will be applied to both immunologically-competent and -incompetent animal models of joint injury to determine the ability of MPCs to repair/regenerate cartilage when protected from pathological levels of pro- inflammatory cytokines. At the end of this project period, we will have a better understanding of how the joint environment - particularly inflammatory signaling cascades - impacts the ultimate success of cartilage regeneration strategies using heterogeneous progenitor cell populations.